The present invention relates to the use of mercapto derivatives as inhibitors of nitric oxide synthase (NOS).
The free radical nitric oxide (NO) is synthesized from the guanidino group of L-arginine by a family of enzymes termed nitric oxide synthase (NOS). The brain isoform (bNOS) is constitutively present in the neural tissue and NO is released as a neurotransmitter by activation of various (e.g. NMDA-type) receptors. NO in the central nervous system plays an important role in the genesis of memory.
The continuous release of NO from the constitutive endothelial isoform of NOS (ecNOS) keeps the vasculature in a continuous state of active vasodilatation and reduces the adhesion of platelets and polymorphonuclear granulocytes (PMNs) to the endothelial surface. The biological activity of NO from the ecNOS was originally described as endothelium-derived relaxing factor (EDRF). The release of EDRF in vivo and in vitro is stimulated by shear stress and various hormones and autocoids such as acetylcholine, bradykinin, substance P. vasopressin, noradrenaline, histamine or platelet-activating factor.
The inducible isoform of NOS (iNOS) is expressed in response to immunological stimuli in multiple cell types including macrophages, vascular smooth muscle cells and epithelial cells, and produces large amounts of NO (nanomoles of NO rather than picomoles of NO derived by the ecNOS or bNOS). NO in high local concentrations can act as a cytostatic and cytotoxic molecule acting against fungal, bacterial, helminthic and protozoal antigens as well as tumor cells. A number of pro-inflammatory cytokines and endotoxin (bacterial lipopolysaccharide, LPS) also induce the expression of iNOS in a number of other cells, including fibroblasts, glial cells, cardiac myocytes as well as vascular and non-vascular smooth muscle cells.
There is now substantial evidence that iNOS plays an important role in the pathogenesis of a variety of diseases. Circulatory shock of various etiologies is associated with profound changes in the body's NO homeostasis. In animal models of endotoxic shock, endotoxin produces an acute release of NO from constitutive isoform of nitric oxide synthase in the early phase, which is followed by induction of iNOS. In addition, it is now thought that excess NO production may be involved in a number of conditions, including conditions that involve systemic hypotension such as septic (toxic) shock and therapy with certain cytokines. Therefore, it is desirable to inhibit nitric oxide synthase. Furthermore, because of the potentially serious consequences of over-inhibition of the constitutive NOS enzyme, it is preferred to selectively inhibit the inducible isoform. Over-inhibition of the constitutive isoform may lead to hypotension, thrombosis, central nervous system toxicity and tissue damage.
Various nitric oxides synthase inhibitors have been proposed for therapeutic use. For example, NG-methyl-L-arginine (L-NMA) and NG-nitro-L-arginine methyl ester (L-NAME) have been suggested, however, they are generally nonselective in that they inhibit both the constitutive and the inducible NOS isoforms to a similar extent. Other NOS inhibitors proposed for therapeutic use include isothiourea derivatives and aminoguanidine. In in vitro and in vivo tests, isothioureas have been shown to inhibit NOS activity, and a few specific compounds within the class of isothioureas have been shown to be relatively selective inhibitors of iNOS activity (see, for example, Garry J. Southan et al., "Isothioureas: Potent Inhibitors of Nitric Oxide Synthases with Variable Isoform Selectivity", British Journal of Pharmacology, Vol. 114, pp. 510-516, 1995; Csaba Szabo et al., "Beneficial Effects and Improved Survival in Rodent Models of Septic Shock with S-methylisothiourea Sulfate, a Potent and Selective Inhibitor of Inducible Nitric Oxide Synthase", Pharmacology, Vol. 91, pp. 12472-12476 (December 1994); and PCT Application No. WO 94/12165). Aminoguanidine also has been found to selectively inhibit the inducible isoform of nitric oxide synthase in various in vitro and in vivo models (see, for example, Chin-Chen Wu et al., "Aminoguanidine Attenuates the Delayed Circulatory Failure and Improves Survival in Rodent Models of Endotoxic Shock", British Journal of Pharmacology, Vol. 113, pp. 001-007, Paper No. 78594 (1995); and European Patent Application 0 547 558 A1).
Although the nitric oxide synthase inhibitors discussed above may prove to have therapeutic use, it is important to identify additional compounds which inhibit nitric oxide synthase. It also is desirable to identify additional compounds which selectively inhibit the inducible isoform of the NOS enzyme. Because excess nitric oxide production plays such a role in a number of different disorders and conditions, such as systemic hypotension, septic shock and cytokine therapy, for example, it is extremely important to identify additional compounds capable of inhibiting, and even selectively inhibiting, nitric oxide synthase. This is especially true given that such additional compounds may prove to have fewer side effects and greater selectivity in inhibiting the inducible nitric oxide synthase enzymes.